1. Field of the Invention
The present invention relates to a control apparatus for controlling a gain to a variable electric-current, and a measuring apparatus having this control apparatus.
2. Description of the Related Art
Hitherto, there has been an industrial machine having a circuit that controls drive electric-current for driving a drive mechanism (see, e.g., JP-A-2004-93181). A surface texture measuring apparatus disclosed in JP-A-2004-93181 has a fine feed mechanism that displaces a measuring element in a fine range, and a coarse feed mechanism that largely displaces the measuring element. The fine feed mechanism is controlled by a fine feed mechanism control circuit. The fine feed mechanism control circuit has two-stages of proportional-plus-integral (PI) circuits (i.e., a first PI circuit and a second PI circuit) that perform two stages of proportional-plus-integral processes, respectively. The first PI circuit and the second PI circuit are configured to have the same configuration. Thus, the first PI circuit and the second PI circuit are set to be the same in time-constant and gain-constant to maximize the responsiveness of each of the circuits.
Meanwhile, the above conventional apparatus is set such that each of a proportional gain and an integral gain of the PI circuit is set at a constant appropriate value, regardless of, e.g., which of a high-speed driving state, a low-speed driving state, and a stopping state the current state of the drive mechanism is. However, if the drive speed of the drive mechanism changes, i.e., if an electric-current for causing the drive mechanism to drive the measuring element is changed, sometimes, the appropriate value differs from the above set appropriate value. Accordingly, the conventional apparatus has a problem in that although an appropriate value is set in the conventional drive mechanism at a constant value by assuming all changes of the electric-current, sometimes, sufficient control performance is not obtained.